1. Field of the Invention
The present invention relates to an antenna apparatus that resonates in a plurality of frequency bands in an inverted F antenna apparatus.
2. Description of the Related Art
Utilization of wireless communications by mobile equipment inclusive of portable telephones as a representative, notebook personal computers and PDA (Personal Digital Assistants) is widespread. Among others, wireless LAN (Local Area Network) attracts attention as one of wireless communication systems. The currently popularized wireless LAN standards include IEEE802.11b/g/n that utilizes the 2.4-GHz band and IEEE802.11a/n that utilizes the 5-GHz band. The 2.4-GHz band is called the ISM (Industry Science Medical) band and utilized for other wireless communications such as Bluetooth (registered trademark) and cordless telephones, microwave ovens and so on, and therefore, interference easily occurs.
On the other hand, since the 5-GHz band also includes a frequency band limited to indoor uses and frequency bands limited in use at the time of radar detection, the 2.4-GHz band and the 5-GHz band are properly used in accordance with the use state. Therefore, developments of wireless equipment and antennas that can cope with both the frequency bands are demanded. Since it is difficult to install a plurality of antennas in the limited casing spaces of portable telephones, PDAs and the like, a dual-frequency shared antenna apparatus that covers the frequency bands of both the 2.4-GHz band and the 5-GHz band with a single antenna apparatus is necessary.
An inverted F antenna has been known as one of the antenna apparatuses that can be small-sized and built-in. As one example of a configuration for resonating the inverted F antenna in two frequency bands, there is an antenna described in a patent document 1 of Japanese patent laid-open publication No. JP 2006-238269 A.
FIG. 11 is a longitudinal sectional view showing a configuration of a prior art dual-frequency resonant antenna apparatus. Referring to FIG. 11, the antenna apparatus is described below by using XY coordinates having a coordinate origin O at one point on the upper surface 104a of a grounding conductor 104. An axis along the upper surface 104a of the grounding conductor 104 is assumed to be an X axis, and an axis extending from the coordinate origin O toward a perpendicular direction (upward direction) from the upper surface 104a of the grounding conductor 104 is assumed to be a Y axis.
Referring to FIG. 11, a first antenna element 101 has a length of λα/4, and resonates at a wavelength of λα. A second antenna element 102 has a length of λβ/4 and resonates at a wavelength of λβ. A Y-direction length long strip ψ is grounded at the coordinate origin O and connected to the first antenna element 101 in the Y-axis direction. A Y-direction short strip y is connected to a feeding point 105, and is connected to the second antenna element 102 in the perpendicular direction.
In the antenna apparatus as configured as above, impedance matching is achieved at feeding points in the 2.45-GHz band and the 5-GHz band by the first antenna element 101 and the second antenna element 102, respectively, and then, a dual-band antenna apparatus is configured. Further, in the patent document 1, frequency band extension is achieved by placing an L-figured passive element 103 between the second antenna element 102 and the upper surface 104a of the grounding conductor 104.
FIG. 12 is a graph showing a frequency characteristics of the voltage standing wave ratio (hereinafter, referred to as VSWR) at the transmission of the dual-frequency resonant antenna apparatus of FIG. 11. As shown in FIG. 12, it can be understood that the frequency characteristic (tuning characteristic) of VSWR changes depending on the length dimension L of the passive element 103 shown in FIG. 11.
The patent document 1 has further had such a problem that a further size reduction is demanded since an antenna apparatus width matched to the longer wavelength is needed due to the parallel arrangement of antenna apparatuses in two rows in the horizontal direction with respect to the grounding conductor in accordance with two wavelengths.